The MEMS (Micro-Electro-Mechanical Systems) mirrors and mirror arrays have wide applications in the light process and fiber optic networks such as in optical cross-connect switches, attenuators, wavelength blocker, dynamic gain equalizer, configurable grating and tunable filter etc. The MEMS mirror arrays with high fill factors and two axes rotation have particular importance in the wavelength division multiplexing systems. Tile fill factor is generally defined as the ratio of the active area to the total area in an array. The high fill factor improves optical channel shape and reduces the optical loss in the system. A micromirror with two axes of rotation can provide switching of the optical beam among the channels while avoiding undesirable optical transient cross-talk during switching, and achieving variable optical attenuations.
There are a number of actuation methods for the MEMS micromirror array such as electromagnetic actuation disclosed in U.S. Pat. No. 6,760,145 B1, thermal actuation disclosed in U.S. Pat. No. 7,091,057 B2, and electrostatic actuation disclosed in U.S. Pat. No. 7,095,546 B2. Electrostatic actuation is favored due to its low power consumption and relative simple structure and small footprint.
Existing micromirrors with electrostatic actuation fall into two categories: vertical combdrive type micromirrors and parallel plate type micromirrors. The drawback for conventional vertical combdrive type micromirrors is its failing to form the high fill factor arrays due to its typical gimbaled and framed structure. Since it is difficult to reduce the gap between adjacent micromirrors, it is hard to form a mirror array with high fill factor. One of these kinds of MEMS micromirrors was disclosed in U.S. Pat. No. 6,822,776 B2.
It is much easier to form high fill factor mirror arrays based on the parallel plate type of actuators. The majority of existing high fill factor micromirror array designs use parallel plate type electrostatic actuators, such as those taught in U.S. Pat. Nos. 7,095,546, 6,934,439, 6,694,073, 6,781,744, 6,778,728, 7,209,274 and 7,053,981. The advantage of using a parallel plate electrostatic actuator is that no typical gimbaled structure or frame is required for the design. As such, the gap between the mirrors can be very small to form a high fill factor mirror array. However, there are several disadvantages for the micromirror array using parallel plate electrostatic actuators.
First of all, the pull-in effect of parallel plate type electrostatic actuator of micromirror limits the controllable tilting angle range under the certain actuation voltage. When an actuation voltage is applied between the fixed electrode and the movable hinged mirror, the resulting attractive electrostatic force will pull the mirror towards the fixed electrode to create tilting of hinged mirror. Initially, the restoring force from deformed hinge will balance the electrostatic force to keep the mirror in the controllable position. But when the actuation voltage is further increased, and the tilting of the hinged mirror is over one third of the initial gap between tie fixed electrode and the mirror, the electrostatic force between the electrode and the mirror surpasses the mechanical restoring force of the hinges, such that the hinged mirror will snap and physically contact die fixed electrode. Thus, the usable and controllable tilting range of the mirror is limited to only one third of the gap between the mirror and fixed electrode. Furthermore, even within the small controllable titling range, the parallel plate electrostatic actuator won't provide linear actuation. In other word, the mirror tilting angle is not linear with the actuation voltage.
Secondly, a high actuation voltage creates issues with respect to electrical charging, tilting angle drifting, and cross-talking between adjacent mirrors. In order to have a larger controllable titling angle for the mirror, the gap between the fixed electrode and the mirror has to be increased. This increased gap results in a higher actuation voltage. Often, several hundreds of volts have to be used to obtain a couple of degrees of mirror titling. Such higher driving voltage causes electrical charging on the dielectrical materials of the mirror device, which will in turn cause the undesired tilting angle drifting of the mirror. Also it is very difficult to shield the electrical field from one mirror actuation electrode to interfere the performance of the adjacent mirrors.
Thirdly, there is a mechanical coupling between the two axes rotation for the micromirror. In the micromirrors disclosed in U.S. Pat. Nos. 7,095,546, 6,934,439, 6,694,073, 6,781,744, 6,778,728, 7,209,274 and 7,053,981, the actuation about one axis rotation will cause the move of the mirror about the other axis. This coupling of two axes rotation makes operation control of the device complicated, and unreliable.
Fourthly, squeezed air between the movable mirrors and the fixed electrodes during tilting will lead to interference among adjacent mirrors. Since the space between the movable mirror and fixed electrode is very small, and the gap between the adjacent mirrors is also small to obtain a high fill factor, the fast titling/switching of one mirror will cause the air film between its mirror and fixed electrode to be either compressed or decompressed. As such, the air flow will be formed. The air flow resulting from the switching of one mirror will therefore interfere with the adjacent mirrors, and cause them to tilt. Furthermore, the air damping from the squeezed air film will effectively lower the switching speed of the mirror.
Lastly, the microfabrication process is costly and complex, especially for making complex actuation electrodes and electrical wirings of two dimensional rotation mirrors.